The present invention relates to addition curing system and more particularly the present invention relates to addition curing systems which have a low viscosity in the uncured state and high physical strength in the cured state.
The addition curing SiH olefin systems are well known. Basically such system comprise a vinyl containing diorganopolysiloxane base polymer, a platinum catalyst and a hydride cross-linking agent which can be either a linear hydride or a hydride containing resin composed of monofunctional units and tetrafunctional units or a hydride resin composed of monofunctional units, tetrafunctional units and difunctional siloxy units. An early patent on such type of compositions is that of Modic U.S. Pat. No. 3,436,366 which discloses the use of a silicone resin composed of monofunctional siloxy units, difunctional siloxy and tetrafunctional siloxy units. This resin has a certain vinyl content and is utilized as an additive to the basic SiH olefin platinum catalyst composition. It has been found that this composition may be useful as both a molding and encapsulating composition. The use of vinyl containing resins permits the lowering of the amount of silica filler there is incorporated in the composition. As a result the composition has good physical strength with a low viscosity in the uncured state making the composition an excellent composition as far as potting and encapsulate uses.
A further modification of such systems can be found in Jeram et al U.S Pat. No. 3,957,713 which is hereby incorporated in the present case by reference. This patent discloses that a high strength silicone composition be obtained by incorporating in the basic vinyl SiH olefin platinum catalyzed systems a low viscosity polysiloxane fluid which is terminated at one end of the polymer chain with triorganosiloxy groups and the other end polymer chain with a vinyl diorganosiloxy groups. It was also disclosed in such systems there could be utilized from 5 to 70 parts of a filler, which is preferably a fumed silica or precipitated silica treated filler. The silica could be either treated with cyclic polysiloxane or silazanes or preferably both. It is necessary to have all these ingredients in the compositions to arrive at a high strength in the cured state that is a tear strength in the 100 to 200 psi range.
Recently, there have been developments in the field of molding compositions, specifically there has been developed liquid injection molding apparatus forming plastic parts. The cost of this apparatus is expensive. However, by the use of this apparatus, molded parts could be formed at a very cheap cost because there is a small amount of material utilized to encapsulate the part or the component, for instance, in the formation of encapsulated parts with little waste, and molding can be done very rapidly to result in the savings of labor costs. There have been attempts to develop silicone compositions for such an application. The difficulty with composition of U.S. Pat. No. 3,957,713 was that the composition did not have a sufficiently low viscosity in the uncured state. For liquid injection molding machines, it is desired that the uncured viscosity be as low as possible, but generally in the range of 10,000 to 500,000 centipoise at 25.degree. C. If the composition has this viscosity, it can be readily forced into the molding chamber of the liquid injection molding machine and then the composition has to have another property, that is, it has to cure at a sufficiently rapid rate. Upon being heated to elevated temperatures, the molded parts can be formed with rapidity. The faster the parts are formed by the liquid injection molding machine, the lower the cost of the part that is formed and the more is the saving in labor that it realized compared to the production of such molded parts by a different method.
For such liquid molding application, it was desirable to develop a silicone composition which could be utilized readily. It was highly desirable to have a silicon composition such as that of Modic which would have a sufficient tear strength in the range of 100 to 200 pi in the cured state and the composition would have a viscosity in the uncured state varying from 10,000 to 500,000 centipoise at 25.degree. C. and more preferably varying from 10,000 to 200,000 centipoise at 25.degree. C. In addition, it is desired that the composition be able to be cured at elevated temperature; that is temperatures above 100.degree. C. in a period of 1 to 10 seconds.
It should be noted as disclosed in Bobear U.S. Pat. No. 4,061,609 which is hereby incorporated by reference that hydroperoxy inhibitors have been developed which can be incorporated to SiH platinum addition systems such that they can be packaged in a single package such that it will not cure at room temperature but would cure rapidly at elevated temperatures about 100.degree. C.
Further, one successful attempt to produce such liquid injection molding compositions was the production of silicone polymers that are fluorosubstituted such that the cured part has good solvent resistance as disclosed in the patent application of Jeram, U.S. Pat. No. 4,041,010 which is hereby incorporated by reference. The patent discloses fluorosilicone compositions comprising as the base fluorosilicone polymer a vinyl fluorosilicone resin combined with a hydride cross-linking agent and a platinum catalyst. The composition disclosed that optionally there may be utilized a filler in the composition; however, in order to get the higher tensile strengths of the composition, it is necessary to incorporate a filler. Accordingly, this composition is not altogether as advantageous as would be desired.
Another disclosure of a fluorosilicone composition which can be utilized in liquid injection molding machines is to be found in Jeram U.S. Pat. No. 4,029,629 which is hereby incorporated by reference. This composition discloses the use of a reinforcing filler in combination with the basic fluorosilicone composition. Accordingly, for such application the composition of U.S. Pat. No. 4,041,010 is much to be preferred over that of U.S. Pat. No. 4,029,629.
Accordingly, it was highly desirable to find some technology whereby there could be obtained a silicone addition cured composition which would have a low viscosity in the uncured state, that is, in the range of 100,000 to 500,000 centipoise at 25.degree. C. and would have a cured tear strength in the range of 100 to 200 pi in the cured state and such that it would have a fast cure rate of 1 to 10 seconds at elevated temperatures, that is, temperatures about 100.degree. C. It is preferred in such a composition in order to keep the viscosity low in the uncured state that the composition contain as low an amount of reinforcing filler as can possibly be used consistent with a high strength in the cured state.
Accordingly, it is one object of the present invention to provide an SiH olefin addition cured system having a high strength in the cured state and a low viscosity in the uncured state.
It is another object of the present invention to provide for an SiH olefin platinum catalyst addition cured system which has a fast cure rate at elevated temperature.
It is still another object of the present invention to provide for an SiH olefin platinum catalyzed addition cured system which is suitable for liquid injection molding applications.
It is still an additional object of the present invention to provide for a process for producing an SiH olefin platinum catalyzed addition cured system which has high strength in the cured state and low viscosity in the uncured state. These and other objects of the present invention are accomplished by means of the disclosure set forth herein below.